New DDE-Linux-based USB driver

The new 'dde_linux' repository will host device drivers ported from the
Linux kernel. In contrast to the original 'linux_drivers' repository,
'dde_linux' does not contain any 3rd-party source code. To download the
Linux kernel source code and extract the drivers, execute the 'make
prepare' rule of the top-level Makefile. The initial version of the
'dde_linux' repository comes with an USB driver. The porting methodology
follows the path of the Intel GEM port. Instead of attempting to provide
a generic Linux environment that works across drivers, each driver comes
with a specially tailored DDE.

The DDE consists of Genode-specific implementations of Linux API
functions as declared in 'lx_emul.h'. Most of these functions are
dummies that must merely be provided to resolve dependencies at the
linking stage. They are called by unused code-paths.

As of now, the USB driver support UHCI, EHCI on the x86_32 platform. I
exposes USB HID devices and USB storage devices via Genode's input-session
and block-session respectively.

The USB driver is accompanied with two run scripts 'run/usb_hid.run' and
'run/usb_storage.run'.

README

@@ -127,10 +127,17 @@ The Genode source tree is composed of the following subdirectories:
   For instructions about how to use this mechanism, please consult the README
   file at the top level of the repository.
 
-:'linux_drivers':
+:'dde_linux':
 
   This source-code repository contains the device driver environment for
-  executing Linux device drivers natively on Genode.
+  executing Linux device drivers natively on Genode. Currently, this
+  repository hosts the USB stack.
+
+:'linux_drivers':
+
+  Deprecated version of the Linux device driver environment featuring audio
+  drivers and the Intel GEM driver. This repository is in a transitionary
+  phase. Its functionality will be incorporated into 'dde_linux'.
 
 :'dde_ipxe':